1. Field of the Invention
The present invention relates to a node for self localization, a clustering method using the same, and a localization method, and more particularly, to a node for self localization based on clusters in a wireless sensor network, a clustering method using the same, and a localization method.
2. Description of the Related Art
Methods of localizing a node in wireless sensor networks are roughly classified into a method of localizing a node using information on a measured distance and a method of localizing a node without information on a measured distance.
In the method of localizing a node using information on a measured distance, location of the node is found by performing triangulation after a distance between a node of which location is to be determined and an anchor node of which location is known.
A distance between nodes is generally measured by a time of arrival (ToA) method, a time difference of arrival (TDoA) method, and a received signal strength (RSS) method.
In the ToA method, a distance is measured by using a time during which a signal with a known transmission speed moves between nodes. In the TDoA method, a distance is measured by using a difference in the times of arrival of the signals by simultaneously transmitting two signals which have different transmission speeds.
When the transmission speed of the signal decreases, and there are no obstacles, the ToA method and the TDoA method can obtain an accurate measurement value.
However, when a signal such as a radio frequency (RF) signal with a high transmission speed is used, it is difficult to measure an accurate difference between distances. In the TDoA method, two signals are used, and therefore additional hardware or an additional sensor is needed.
In addition, when a signal such as an ultrasonic wave or sound wave with a high transmission speed is used for ToA and TDoA methods, it is difficult to secure against line of sight (LoS). Since the ToA and TDoA methods are largely influenced by indoor obstacles, it is difficult to communicate each other.
In the RSS method, a distance is measured by using the intensity of a signal that arrives at a node. The RF signal used for the RSS method has a good diffraction property and secures against LoS without additional hardware, but hardly measure an accurate distance, too.
In general, the RSS method has a low degree of accuracy for measuring distance. The RSS method is largely influenced by indoor obstacles such as walls or furniture.
In an angle of arrival (AoA) method, the location of a node is determined by using an angle between two nodes which communicate with each other. In the AoA method, in order to find an angle, a ToA or RSS value is converted into an angle by using a multi-antenna.
However, it is difficult to construct hardware for mounting the multi-antenna on the general node. The size of the node increases, and accordingly AoA is not generally used.
The localization method without distance information includes a centroid method and an approximate point in triangulation (APIT) method. The localization method without distance information is used for preventing an error from spreading in the sensor network which constitutes a multi-hop network.
In the centroid method, regularly arranged anchor nodes transmit their own location information to neighboring nodes, and the nodes estimate their own locations by comparing the intensities of the signals received from the anchor nodes.
In the centroid method, as the anchor nodes are regularly arranged, the number of anchor nodes which can communicate with the node increases, and the RF transmission environment is similarly maintained, it is possible to measure the location of the node. Accordingly, the centroid method is not suitable for indoors.
In the APIT method, the location of the node is estimated by determining whether the node exists in the triangle constructed by anchor nodes. In the APIT method, the location of the node is estimated by also using the intensity of the signal.
As described above, in the conventional localization method, it is difficult to accurately localize the node, because the signal error caused by obstacles is included in a triangulation value in indoor environments where there are many obstacles.